Previous control systems of the analog type required the use of components such as moving parts, bridge circuits, moving contacts, electro-mechanical relays, and conductive containers, and have not been isolated from the power line. Their performance, safety, reliability and usefulness has been limited as a result.
Whereas analog control systems depend on accurate measurement of critical voltage levels and are subject to error and noise, digital control devices use signals or no signal, in digital parlance "one" which is above 70% of supply voltage or "zero", which is less than 30% of supply voltage and have largely supplanted analog systems or devices in all fields of computation and control due to their characteristics of unambiguity, accuracy, reliability, freedom from noise, flexibility, and versatility of design. The advent of solid state digital integrated circuits and solid state relays into the electronic field has furnished new components of greater reliability, safety, longer life, and greater versatility and have been utilized in the present invention.
The present invention takes advantage of these superior digital characteristics and employs level sensors which send D.C. digital sensing signals to a digital signal processing circuit which processes these signals according to state and sequence. The digital logic processing circuit output signal is used to actuate a solid state relay which can control either A.C. or D.C. power and which provides complete isolation of the sensing and logic circuits from the power line. The digital logic signal processing circuit inputs can be logic gates, for example, of very high resistance which enables sensing of direct current digital signals of a small fraction of a millionth of an ampere through conducting fluent material of very high resistance thus extending the scope of control to very high resistance types of material. Use of small currents, low D.C. voltages and small connecting wires to the sensors make the present invention suitable for use with battery power supply and remote sensing, an adaptability lacking in previous devices. The two level sensors of the present invention can be placed in any environment and need no conductive reservoir for operation. The present invention can be used with a large assortment of electrically conductive granular or fluent materials by the use of appropriately designed electrodes for the sensors. The present invention is further applicable to a large variety of electrically non-conductive and granular solid or opaque materials by the use of appropriately designed transducers used as sensors which are capable of delivering a D.C. logic signal "one" when immersed in the fluent material and a "zero" when out of the material.